A vertical cavity surface emitting laser (VCSEL) is a semiconductor laser including a semiconductor layer of optically active material, such as gallium arsenide or indium phosphide. The optically active material is sandwiched between mirror stacks formed of highly-reflective layers of metallic material, dielectric material, or epitaxially-grown semiconductor material. Conventionally, one of the mirror stacks is partially reflective so as to pass a portion of the coherent light which builds up in a resonating cavity formed by the mirror stacks sandwiching the active layer. At least one of the mirror stacks has a reflectivity greater than approximately 99.5%
Lasing structures require optical confinement in the resonating cavity and carrier confinement in the active region to achieve efficient conversion of pumping electrons into stimulated photons through population inversion. The standing wave of reflected optical energy in the resonating cavity has a characteristic cross-section giving rise to an optical mode. A desirable optical mode is the single fundamental transverse mode, for example, the HE.sub.11 mode of a cylindrical waveguide. A single mode signal from a VCSEL is easily coupled into an optical fiber, has low divergence, and is inherently single frequency in operation.
In order to reach the threshold for lasing, the total gain of a VCSEL must equal the total loss of the VCSEL. Unfortunately, due to the compact nature of VCSELs, the amount of gain media is limited. Because epitaxially-grown mirror stacks often do not enable sufficiently high reflectivity, some vertical cavity surface emitting lasers are formed by wafer fusing the top and bottom mirror stacks to the active region. Wafer fusion is a process by which materials of different lattice constant are atomically joined by applying pressure and heat to create a real physical bond. Thus, wafer fusion of one or both of the mirror stacks to the active region is used to increase the reflectivity provided by either or both of the mirrors to compensate for the small amount of gain media so that the lasing threshold can be reached and maintained.
An important requirement for low-threshold, high- efficiency VCSEL operation is a lateral refractive index variation or index guiding mechanism which introduces low optical loss for the VCSEL. Lateral oxidation of AlGaAs has been used for refractive index guiding to make high-efficiency VCSELs. In such a lateral oxidation technique, a mesa is etched into the top surface of the VCSEL wafer, and the exposed sidewalls of an AlGaAs layer are exposed to water vapor. Water vapor exposure causes conversion of the AlGaAs to AlGaO.sub.x, some distance in from the sidewall toward the central vertical axis, depending on the duration of oxidation. This introduces a lateral refractive index variation, creating a low-loss optical wavequide if the AlGaO.sub.x layer is sufficiently thin. Shortcomings of this approach are that it leads to a non-planar structure, since mesa etches are required, and the oxidized AlGaAs expands, producing a fragile mesa. The reliability of VCSELs using this technique is unknown.